In U.S. Pat. No. 4,428,647, Sprague et al. describe an optical system, such as a multi-channel optical disk recorder or a laser printing system, having a laser array with a plurality of laser sources providing a plurality of output light beams. Each of the laser sources in the array has its own current drive which can be modulated, independently of the other sources, dependent on the desired output power of the emitted light beam from that source. The optical system also includes a focusing objective lens which images the plural light beams onto the surface of a photosensitive medium. The objective lens is located a substantial distance from the emitting surface of the laser array relative to its distance to the photosensitive medium to provide enough demagnification of the array of light beams that the spacings of the imaged light spots on the medium are substantially less than the spacings of the light beams at the laser array. A 125-fold reduction in the light spot spacings from 250 .mu.m at the laser array emitting surface to just 2 .mu.m at the medium is typical. Even overlapped or concentric light beams are suggested as a possibility. Due to the point source nature of the light beams emitted from the described array, those beams typically have a large divergence angle. In order to enable a large portion of each of the light beams to be collected by the objective lens so that light spots of significant power are imaged onto the photosensitive surface, the optical system further includes an array of lenses, one for each laser source in the laser array, located in the optical path between the laser array and objective lens. Each lens in the lens array reduces the divergence angle of the light beam received from its associated laser source so that substantially all of the light collected by each lens will enter the objective lens without changing the apparent beam spacing of the laser array seen by the objective lens. Lenses with cylindrical symmetry may be used to shape the individual laser beams and to compensate for laser astigmatism.
U.S. Pat. Nos. 4,972,427 to Streifer et al., 5,081,637 and 5,185,758 to Fan et al., and 5,168,401 to Endriz disclose other laser optical system of the prior art which position lens arrays in front of corresponding arrays of laser diodes. In the patents to Fan et al., the collimated light emerging from the lens arrays are focused by a lens in order to converge the beams so that they overlap in a solid-state gain medium for optically pumping that medium. Streifer et al. place a lens array within an external Talbot cavity. A separate cylindrical collection lens is use to reduce the divergence in the transverse direction perpendicular to the plane of the laser diode emitters, while the individual lenses of the lens array collimate the light beams in the lateral direction parallel to the plane of the array. Endriz combines a lens array with a corresponding array of turning mirrors which transform the lateral and transverse dimensions of each source in the laser array. The lens elements in the lens array are preferably positioned such that the light beams are allowed to diverge to completely fill the space between the transformed light from adjacent sources. Each light source in the array may be separately modulated.
In U.S. Pat. No. 4,674,095, Heinen et al. describe a laser diode array provided with a plurality if laser diode strips. The laser diode strips are parallel to each other and are divided into groups. Typical strips are 2 to 4 .mu.m wide and are spaced about 10 to 20 .mu.m apart. Neighboring groups of laser diode strips are separated by strip-shaped zones extending essentially parallel to the laser diode strips. The zones substantially attenuate super-radiation or laser radiation propagating in a direction other than the prescribed emission direction parallel to the laser diode strips. Up to a maximum of about 10 to 40 laser diode strips can belong to each group, depending on the gain in the laser, the quality of the resonator and other parameters. The attenuating zones may be constructed by proton implantation, channels etched through the active region, or other means. The partitioning provided by the attenuating zones increases the maximum output power attainable from the array and improves efficiency.
Among the commercially available laser diodes, the 2300 series of laser sold by SDL, Inc. provides up to 4 W cw optical output power and high brightness from laser diodes with a broad area emitting aperture. For example, the SDL-2360 has a 100 .mu.m wide by 1 .mu.m high emitting aperture and provides a 1.2W cw output with a 10.degree. by 30.degree., FWHM beam divergence. The available wavelength range is approximately 790 to 860 nm. The laser output is modulatable with rise and fall times of about 500 ps (2 GHz modulation bandwidth). Among the linear array laser diodes sold by SDL, Inc. are the SDL-3400 series of laser bars. For example, the SDL-3460 has 18 laser emitters driven in parallel and providing up to 20W cw optical output power or about 1.1W per emitter. Each emitter has a 200 .mu.m wide emitting aperture with a 30 .mu.m gap centered within the emitting aperture. The emitters in the array have a 540 .mu.m center-to-center spacing. The beam divergence is 10.degree. by 30.degree., FWHM.
An important concern of manufacturers of laser diodes is improving the reliability of their products. In the case of broad area lasers, a local failure in any portion of the emitting aperture will tend to grow and propagate across the entire aperture area, resulting in a complete failure of the emitter. In the case of laser arrays, particularly those arrays whose elements are intended to be independently driven, as in laser printing systems, the loss of a single emitting element will normally leave the entire array unusable. That is, in many laser diode applications the loss of just a single laser element cannot be tolerated. For large aperture lasers and multi-element arrays of such lasers, the probability of a single failure point taking out the entire aperture or array increases with increased aperture area. Further, at high power density, stochastic (random) failures are dominant, making life predictions a statistical process and making screening of such failures nearly impossible. The effect of gradual degradation processes on lifetimes at most power levels of interest is not significant compared to stochastic failure processes. Typical broad area lasers with a 100 .mu.m wide emitting aperture operated at a 1.0W optical output power have a medium lifetime on the order of 30,000 hours. Median failure rates for each element in 10-element laser arrays are comparable. However, since stochastic failures are essentially random in time, many of the laser elements will fail much sooner than the 30,000 hour average. Accordingly, median lifetime for a multi-element array of 1 Watt, 100 .mu.m aperture elements will be much less than the element lifetime of 30,000 hours. Thus, improved lifetimes are sought. Because failure rates are correlated with operating power and temperature, due to thermal stresses resulting from temperature gradients across a device, a common technique for improving lifetimes is to rate the device for lower power levels and temperature ranges, so that less thermal stress occurs. However, it would be desirable to improve reliability without having to sacrifice output power and brightness or their use in extreme temperature environments.
An object of the present invention is to provide a diode laser source with significantly improved lifetime for a given output power, output brightness (W cm.sup.-2 sr.sup.-1) and operating temperature rating.